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Abstract:

A method for treating lignin-containing fibrous material to reduce its
susceptibility to yellowing generally includes enzymatically stabilizing
the lignin of the material with an oxidizing agent capable of oxidizing
phenolic or similar groups, which may undergo reactions conductive to the
formation of colored sites on the fibers, and treating the material with
a fluorescent whitening agent. Also disclosed are lignin-containing
materials obtained by the method.

Claims:

1. A method for treating lignin-containing fibrous material to reduce its
susceptibility yellowing, comprising: enzymatically stabilizing the
lignin of the material with an oxidizing agent capable of oxidizing
phenolic or similar groups, which may undergo reactions conducive to the
formation of colored sites on the fibers; and treating the material with
a fluorescent whitening agent.

3. The method of claim 1, wherein the fibrous material is treated with
the fluorescent whitening agent after the stabilization.

4. The method of claim 1, wherein the oxidizing agent is selected from
peroxidases and oxidases.

5. The method of claim 1, wherein the oxidizing agent is selected from
the group consisting of laccases, catechol oxidases, tyrosinases,
bilirubin oxidases, horseradish peroxidase, manganese peroxidase and
lignin peroxidase.

6. The method of claim 1, wherein the fluorescent whitening agent is a
compound of formula (I): ##STR00002## wherein n is an integer number
from 0 to 2, M is an alkali metal ion or optionally substituted ammonium
ion, and X is N-alkylamino or N,N-dialkylamino, where the alkyl radicals
in the combined terms N-alkylamino and N,N-dialkylamino are to be
understood as meaning those having up to 4 carbon atoms, which may be
interrupted by an O atom and/or may carry, as a substituent, hydroxyl,
carbamoyl, cyano or sulfo, and when it is N,N-dialkylamino, the two alkyl
radicals which are optionally interrupted by a heteroatom selected from
O, N and S, together with the N-atom to which they are bonded may form a
saturated 5- or 6-membered heterocycle.

7. The method of claim 1, wherein prior to enzymatically stabilizing the
lignin of the material, the material is pretreated with a reductive
agent.

8. The method of claim 7, wherein the reductive agent is selected from
the group consisting of boron hydride, dithionite, bisulfate, sulfur
dioxide water, and mixtures thereof.

9. The method of claim 1, wherein prior to enzymatically stabilizing the
lignin of the material, the material is further treated with a modifying
agent to block reactivity of oxidized sites in the material.

10. The method of claim 9, wherein the modifying agent is a brightness
reversion inhibitor.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to a method for treating
lignin-containing fibrous material to reduce its susceptibility to
yellowing. More particularly, the present invention relates to such
method comprising treating the material with a fluorescent whitening
agent.

BACKGROUND OF THE INVENTION

[0002] It is well-known in the art that light (UV light in particular),
heat, moisture, and chemicals can give rise to changes in the brightness
of lignin-containing material, such as cellulose pulps. Usually such
changes result in reduced reflectivity, particularly in the blue light
region. This phenomenon is known as brightness reversion or yellowing and
can be caused by various factors depending on which type of
lignin-containing material is concerned. Heat and moisture are the main
causes of the brightness reversion of chemical (lignin-free) pulps,
whereas mechanical pulps mostly yellow when they are exposed to light.
The brightness reversion of mechanical pulps also varies depending on the
raw material (type of wood), production method (with or without chemical
pretreatment) and after-treatment (bleaching with different reagents)
used. Thus, for instance, sulfonation and peroxide bleaching greatly
increase the susceptibility of pulp to light-induced yellowing.

[0003] The brightness reversion of lignocellulosic materials, such as
pulps, and product made from such material, can be reduced in various
ways, e.g. by means of impregnation of surface treatment using UV
screens, antioxidants or polymers, or by coating the surface with a
coating layer or a layer of non-yellowing chemical pulp. Various
additives are described in patent literature. For example, U.S. Pat. No.
4,978,363 discloses a composition and method for treating fibers based on
a mixture of an organopolysiloxane having at least one amino-substituted
hydrocarbon radical directly bonded to a silicon atom and a higher fatty
carboxylic acid. The carboxylic acid reacts with the amino radicals to
reduce yellowing and oxidation of the fiber treatment.

[0004] U.S. Pat. No. 6,599,326 discloses inhibition of pulp and paper
yellowing using hydroxylamines and other coadditives. Chemical pulps and
papers, especially kraft pulps and papers, which may still contain traces
of lignin, have enhanced resistance to yellowing when they contain an
effective stabilizing amount of an N,N-dialkylhydroxylamine, an ester,
amide or thio substituted N,N-dialkylhydroxylamine or
N,N-dibenzylhydroxylamine or an ammonium salt thereof.

[0005] WO 2005/061782 discloses a process for producing a fiber material
having reduced susceptibility to yellowing comprising activating the
fibers of the matrix with an oxidizing agent capable of oxidizing
phenolic or similar structural groups, which may undergo reactions
conducive to the formation of colored sites on the fibers, and attaching
to the oxidized sites at least one modifying agent to block the
reactivity of the oxidized sites.

[0006] Many of the additives that have been found to prevent yellowing are
expensive or problematic from an environmental point of view. Some are
only effective when introduced in amounts so large that they may have a
negative effect on other properties of the product or be uneconomical.
Accordingly, there is still a need for methods for preventing yellowing.

SUMMARY OF THE INVENTION

[0007] It is an aim of the present invention to eliminate the problems of
the prior art and to provide new methods for reducing or preventing
yellowing. The methods aim at effectively reducing light-induced
brightness reversion of lignin-containing fibrous materials, such as
pulps.

[0008] It was surprisingly found out that use of the modifying agent as
disclosed in WO 2005/061782 is not necessarily required but that the use
of an oxidizing agent alone is enough to stabilize the lignin.
Furthermore, it was discovered that when the lignin-containing material
was further treated with a fluorescent whitening agent after the
stabilization, it provided an advantageous synergic effect and reduced
the oxidizing-agent-based drop in initial brightness. Lignin structure
seems to be modified in such a way that unfavorable side reactions are
reduced.

[0009] The present invention provides a method for treating
lignin-containing fibrous material to reduce its susceptibility to
yellowing, comprising stabilizing the lignin of the material with an
oxidizing agent capable of oxidizing phenolic or similar groups, which
may undergo reactions conductive to the formation of colored sites on the
fibers, and treating the material with a fluorescent whitening agent.

[0010] The present invention also provides a lignin-containing material
obtained by said method.

[0014] The lignin-containing materials usually contain a fiber matrix
comprising fibers containing phenolic or similar structural groups, which
are capable of being oxidized by suitable oxidizing agents. Such fibers
are typically "lignocellulosic" fiber materials, which include fiber made
of annual or perennial plants or wooden raw material by, for example,
mechanical, chemimechanical or chemical pulping. During industrial
refining of wood by, e.g., refiner mechanical pulping (RMP), pressurized
refiner mechanical pulping (PRMP), thermomechanical pulping (TMP),
groundwood (GW) or pressurized groundwood (PGW) or chemithermomechanical
pulping (CTMP), a woody raw material, derived from different wood species
as for example hardwood and softwood species, is refined into fine fibers
in processes, which separate the individual fibers from each other. The
fibers are typically split between the lamellas along the interlamellar
lignin layer, leaving a fiber surface which is at least partly covered
with lignin or lignin-compounds having a phenolic basic structure

[0015] Within the scope of the present invention, also chemical pulps are
included if they are susceptible to brightness reversion and have a
residual content of lignin sufficient to give at least a minimum amount
of phenolic groups necessary for providing binding sites for the
modifying agent. Generally, the concentration of lignin in the fiber
matrix should be at least 0.1 weight percent (wt-%), preferably at least
about 1.0 wt-%.

[0016] Onefeature of the invention is to block brightness reversion by
modifications of phenolic hydroxyls, alpha-carbonyls and/or
alpha-hydroxyls on the fibers. In particular, by subjecting lignin
structures to enzymatic oxidation to yield oxidized groups of the
afore-said kind, the normal reactions causing brightness reversion can be
attained.

[0017] In the method of the present invention the lignin-containing
material is stabilized with an oxidizing agent capable of oxidizing
phenolic or similar groups, which may undergo reactions conductive to the
formation of colored sites on the fibers. The stabilization is directed
to the lignin and may be carried out enzymatically or chemically. In the
stabilization OH-groups are formed which stabilize the structure and
prevent the yellowing. In other words, the parts causing the yellowing
are deactivated.

[0018] Typically, the stabilizing agent is an enzyme and the enzymatic
reaction is carried out by contacting the lignin-containing material with
an oxidizing agent, which is capable--in the presence of the enzyme--of
oxidizing the phenolic or similar structural groups to provide oxidized
lignin-containing material. Such oxidizing agents are selected from the
group of oxygen and oxygen-containing gases, such as air, and hydrogen
peroxide. Oxygen can be supplied by various means, such as by efficient
mixing, foaming, gases enriched with oxygen or oxygen supplied by
enzymatic or chemical means, such as peroxides to the solution. Peroxides
can be added or produced in situ.

[0019] According to one embodiment of the invention, the oxidative enzymes
capable of catalyzing oxidation of phenolic groups are selected from e.g.
the group of phenol oxidases (E.C.1.10.3.2 benzenediol:oxygen
oxidoreductase) and catalyzing the oxidation of o- and p-substituted
phenolic hydroxyl and amino/amine groups in monomeric and polymeric
aromatic compounds. The oxidative reaction leads to the formation of
phenoxy radicals. Other groups of enzymes comprise peroxidases and other
oxidases. "Peroxidases" are enzymes which catalyze oxidative reaction
using hydrogen peroxide as their electron acceptor, whereas "oxidases"
are enzymes which catalyze oxidative reactions using molecular oxygen as
their electron acceptor.

[0021] The amount of the enzyme is selected depending on the activity of
the individual enzyme and the desired effect on the lignin-containing
material. Advantageously, the enzyme is employed in an amount of
0.0001-10 milligrams (mg) protein/gram (g) of dry matter
lignin-containing material.

[0022] Different dosages can be used, but advantageously a dosage of about
1 to about 100 000 nkat/g, and in other embodiments, 10 to 500 nkat/g is
sufficient.

[0023] In addition to enzymes, also chemical agents, such as alkali metal
persulfates and hydrogen peroxide and other per-compounds, can be used
for achieving oxidization of the phenolic groups and for forming phenoxy
radicals. The dosage of the chemical agent is, depending on the chemical
agent and the lignin-containing material (i.e. on the amount of phenolic
groups contained therein), typically in the range of about 0.01 to about
100 kg/ton, preferably about 0.1 to about 50 kg/ton, e.g. about 0.5 to
about 20 kg/ton. In the case of chemical agents, no separate oxidation
agent needs to be added. The per-compound will achieve the aimed
oxidation of the phenolic groups.

[0024] The stabilization treatment is carried out in a liquid medium,
preferably in an aqueous medium, such as in water or an aqueous solution,
at a temperature in the range of 5 to 100° C., typically about 10
to 85° C. Typically, a temperature of 20 to 80° C. is
preferred. The consistency of the pulp is, generally, 0.5 to 95% by
weight, typically about 1 to about 50% by weight, in particular about 2
to about 40% by weight. The pH of the medium is preferably slightly
acidic; in particular the pH is about 2 to 10 at room temperature in the
case of phenol oxidases. The chemical agents are usually employed in
slightly acidic conditions, such as at pH 3 to 6. Peroxidases are
typically employed at pH of about 3 to 12. The reaction mixture is
stirred during oxidation. Other enzymes can be used under similar
conditions, preferably at pH 2 to 10.

[0025] In the method of the present invention the material is further
treated with a fluorescent whitening agent (FWA). In one embodiment the
fluorescent whitening agent is a compound of the formula (I):

##STR00001##

wherein n is an integer number from 0 to 2, M is an alkali metal ion or
optionally substituted ammonium ion, and X is N-alkylamino or
N,N-dialkylamino, where the alkyl radicals in the combined terms
N-alkylamino and N,N-dialkylamino are to be understood as meaning those
having up to 4 carbon atoms, which may be interrupted by an O atom and/or
may carry, as a substituent, hydroxyl, carbamoyl, cyano or sulfo, and
when it is N,N-dialkylamino, the two alkyl radicals which are optionally
interrupted by a heteroatom selected from O, N and S, together with the
N-atom to which they are bonded may form a saturated 5- or 6-membered
heterocycle.

[0026] Generally FWA is added to pulp or paper machine wet-end as an
aqueous solution of active molecule (such as the one represented by
formula (I)), which may include some additives (e.g., to improve
solubility or performance) or it may just be FWA-water solution as such.
This is known as "FWA formulation". In the method of the present
invention, the lignin-containing material may be treated with a
fluorescent whitening agent or any suitable formulation thereof.

[0027] Also a special pretreatment step may be combined with the
stabilization and FWA treatment. When the lignin-containing material is
pretreated with a reducing agent before the stabilization, it provides an
advantageous synergic effect and reduces the oxidizing-agent-based drop
in initial brightness. Lignin structure seems to be modified in such a
way that unfavorable side reactions are reduced.

[0028] In such embodiment the lignin-containing material is pretreated
with a reducing agent. Examples of suitable reducing agents include boron
hydride, such as sodium boron hydride (sold e.g., by trade name
Borino® by Finnish Chemicals Oy), dithionite (hydrosulfite),
bisulfate, sulfur dioxide water or mixtures thereof. The reducing agent
does not particularly act as a bleaching chemical at this step but acts
more as a fiber modification agent.

[0029] The method of WO 2005/061782 may also be applied to the present
invention. In such a case, after the stabilization the material is
further treated with a modifying agent to block the reactivity of the
oxidized sites. In one embodiment the modifying agent is a brightness
reversion inhibitor. The modifying agent has at least one functional site
or reactive structure which provides for binding of the modifying
compound to the lignocellulosic material, in particular in the oxidized
phenolic groups or corresponding chemical structures of the
lignin-containing material, which have been oxidized during the
stabilization step.

[0030] The modifying agent can be an aliphatic or aromatic, monocyclic,
bicyclic or tricyclic substance. The aliphatic compound can be an
unsaturated carboxylic acid, advantageously a monocarboxylic unsaturated
fatty acid, having 4 to 30 carbon atoms. In particular, the modifying
agent can be a monocarboxylic, unsaturated fatty acid containing a
minimum of two double bonds, preferably two conjugated double bonds. Such
fatty acids have an even number of carbon atoms, typically in the range
of 16 to 22. It is also possible to use lower alkanols, i.e. alcoholic
compounds comprising 1 to 6, in particular 1 to 4 carbon atoms. Examples
include n- and i-propanol and n- and t-butanol.

[0031] Examples of particularly suitable compounds are constituted by
linoleic and linolenic acid. It would appear that the unsaturated fatty
acid bonds to the oxidized groups or structure via one of the double
bonds. In one embodiment, linoleic acid (LA) is used, preferably in
combination with activation carried out by using laccase enzyme.

[0032] Other suitable compounds include antioxidants, such as tocopherol
and beta-carotene. The compound can have special properties, such as
capability to trap radicals and form colorless substituents.

[0033] After the above processing, the modified lignin-containing material
having new and improved properties is generally separated from the liquid
reaction and further used in target applications, such as high quality
consumer packaging and graphic papers.

[0034] The following non-limiting examples illustrate the invention.

EXAMPLES

Example 1

[0035] The treatments were started by cold disintegration of peroxide
bleached aspen/spruce CTMP pulps. The pulps were additionally washed
twice with water (80° C.) after the disintegration. The bonding
was started by mixing 5 g of o.d. pulp with water, the pH of the pulp
slurry was adjusted to pH 7. Thereafter, laccase (Trametes Hirsuta) was
added (10 nkat/g). Laccase induced activation time was 1 min at
55° C. The linoleic acid (LA) was dissolved first in 1 ml of
acetone and then added to the pulp slurry dropwise. Mixing time after
addition of the LA was 39 min (55° C.). The dosage corresponded to
0.075 mmol linoleic acid/g pulp. The total treatment time was 40 min.
After the treatment the pulp was filtrated twice and washed with water
(with an amount equal to 20× dry weight).

[0036] After the enzymatic treatment the pulp was suspended into distilled
water at a consistency of 0.625%. Fluorescent whitening agent (FWA) was
diluted to a concentration of 0.5% and then added to pulp slurry at the
desired final concentration (5 kg/to.d. pulp). After addition pulp
was mixed for 10 min at RT covered from day light by aluminium foil and
black plastic bag.

[0037] The reference treatment was performed with identical procedure, but
without the addition of the enzyme, LA or FWA.

[0038] After all treatments the pulps were mixed in water in a
concentration of 5 g/l and disintegrated 5000 revs before preparation of
two handsheets/treatment on wire cloth according to SCAN M 5:75.

[0039] Aspen BCTMP shows clear indications of light induced yellowing when
subjected to light irradiation by Xenotest device (FIG. 1). When pulp is
modified by laccase (ThL) and further treated with LA, the brightness
stability measured as delta brightness is improved but the initial
brightness drops severely. Addition of FWA (5 kg/t as a product,
Blankophor DS) raised the ISO brightness very close to the original
value. The light stability also stays at a very good level compared to
reference pulp. In this sense FWAs can also be considered to counteract
the detrimental effect of brightness drop by laccase in general.

Example 2

[0040] The treatments were started by reductive treatment of the peroxide
bleached aspen/spruce CTMP pulps. Pulps were diluted to the consistency
of 10%, tempered to 60° C. prior to addition of Borino®.
Charge of Borino was 0.1% and treatment time 3 minutes. During treatment
pH was controlled to be >9. After treatment pulps were diluted with
fresh water and washed twice with water.

[0041] The pulps were additionally washed twice with water (80° C.)
after the disintegration. The bonding was started by mixing 5 g of o.d.
pulp with water, and the pH of the pulp slurry was adjusted to pH 7.
Thereafter laccase (MaL) was added (10 nkat/g). Laccase induced
activation time was 1 min at 55° C. The linoleic acid (LA) was
dissolved first in 1 ml of acetone and then added to the pulp slurry
dropwise. Mixing time after addition of the LA was 39 min (55°
C.). The dosage corresponded to 0.075 mmol linoleic acid/g pulp. The
total treatment time was 40 min. After the treatment, the pulp was
filtrated twice and washed with water (with an amount equal to 20×
dry weight).

[0042] After the enzymatic treatment the pulp was suspended into distilled
water at a consistency of 0.625%. Fluorescent whitening agent (FWA) was
diluted to a concentration of 0.5% and then added to pulp slurry at the
desired final concentration (5 kg/to.d. pulp). After addition, pulp
was mixed for 10 min at RT covered from day light by aluminium foil and
black plastic bag.

[0043] The reference treatment was performed with identical procedure, but
without the addition of the enzyme, LA or FWA.

[0044] After all treatments the pulps were mixed in water in a
concentration of 5 g/l and disintegrated 5000 revs before preparation of
two handsheets/treatment on wire cloth according to SCAN M 5:75.

[0045] As seen previously, aspen BCTMP shows clear indications of light
induced yellowing when subjected to light irradiation by Xenotest device.
When pulp is modified by laccase and LA treatment and further treated
with FWA (5 kg/t as a product, Blankophor DS) good brightness stability
can be achieved (FIG. 1). The effect can be further enhanced by a
reductive treatment prior the laccase modification. FIG. 2 clearly shows
how the Borino treated pulp responses very well to the above-mentioned
treatment.